Hardware equipment and other articles often require cleaning during manufacture, prior to transportation or shipment, after use, and prior to cleanroom entry. The purpose of the cleaning process is to remove particulate matter and other contaminants present on the surface of the article in order to prevent contamination of other surfaces in the cleanroom environment.
Solvent wipe and gas blow-off techniques are examples of conventional cleaning processes. For example, a solvent wipe may include physical contact of a low-linting cloth or fiber wipe (e.g., moistened with a solvent such as isopropyl alcohol). For items with complex surface geometries, compressed air or dry nitrogen may be flowed over the surface to blow off contaminants.
Both solvent wipe and gas blow-off techniques have disadvantages with respect to the removal of particulate contaminants. Representatively, since solvent wiping is a contact cleaning method, there is a high risk of damage to sensitive components or delicate surfaces. Gas blow-off techniques generally remove larger particles, but typically will not remove particles smaller than about 2 microns due to boundary layer effects. Additionally, both solvent wipe and gas blow-off are tedious and difficult for operators to perform effectively on large equipment surfaces.
An alternative, non-contact cleaning technique involves the use of carbon dioxide (CO2) snow cleaning. In this method, liquid CO2 is flowed under high pressure through a small orifice positioned to face the item to be cleaned. The resulting pressure differential forces the liquid CO2 to transition from the liquid to the solid phase by operation of Joule-Thompson cooling.
The relationship between temperature, pressure and volume of a gas is generally described by the gas laws. When volume is increased, the gas laws do not uniquely determine what happens to the pressure and temperature of the gas. In general, when a gas expands adiabatically, the temperature may either decrease or increase, depending on the initial temperature and pressure. For a fixed pressure, a gas has a Joule-Thomson (Kelvin) inversion temperature, above which expansion causes the temperature to rise, and below which expansion causes cooling. For most gases, at atmospheric pressure this temperature is fairly high (above room temperature), and so gases may be cooled by expansion.
In accordance with this procedure, CO2 snowflakes may be produced in the 5 micron range for aggressive cleaning as well as up to about 0.5 cm for the cleaning of delicate surfaces. Control of the size of the CO2 snowflakes may be accomplished by varying the flow rate through the nozzle. As CO2 snowflakes impinge on a surface, they transfer momentum to particulate matter. When the CO2 snowflakes sublime, particulate contamination is generally carried away from the surface, thus cleaning the surface.
This form of cleaning is able to achieve a higher level of cleanliness than simply blowing a gas, such as dry air or nitrogen, over a surface. The carbon dioxide flakes are able to penetrate the boundary layer and efficiently remove sub-micron contaminants down to 0.1 microns in size. Since CO2 snowflakes sublime upon impingement on a surface, substantially no residue is left on the surface after cleaning.
The benefits of the CO2 snow cleaning technique are that it is a non-contact method, thereby reducing the risk of damage to sensitive surfaces. Additionally, CO2 snow cleaning removes very small (e.g., sub-micron) contaminants. Moreover, CO2 snow cleaning is appropriate for the removal of light hydrocarbons. For example, a thin layer of liquid CO2, formed at the interfaces between the CO2 snow particle and the surface, may act as a solvent by dissolving organic contaminants and lifting them away from the surface in the flow of CO2 snow and vapor.
Conventional CO2 snow cleaning equipment generally consists of hand-held spray guns with hose attachments to a CO2 liquid source. The operator performing the cleaning must generally hold the spray gun and control the flow of CO2 snow over the surface to be cleaned. For larger pieces of hardware, cleaning with a CO2 snow gun may be difficult, since only a small surface area at a time may typically be cleaned. In these situations, cleaning with a single CO2 snow gun may be time consuming, and it may be difficult to identify which surfaces have already been cleaned and which surfaces have yet to be cleaned.
In another conventional application, CO2 snow cleaning may be performed within a manual glove box. An operator must generally fit gloved hands into the glove box and manually orient the surface of the article to be cleaned with one hand while controlling the CO2 snow gun with the other hand. This reduces the non-contact aspect of CO2 snow cleaning, and is generally not effective for cleaning larger hardware articles and surfaces.